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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 150621

Area Forecast Discussion
National Weather Service Honolulu HI
821 PM HST Fri Dec 14 2018

Mostly dry and pleasant weather can be expected over the weekend
thanks to a stable trade wind air mass. Winds will gradually
diminish over the weekend and on into early next week as the high
weakens, due to a front approaching the islands. The front is
expected to move down the island chain from late Tuesday through
Wednesday with showers increasing with its passage. Cooler and
breezy weather will fill in behind the front.


The forecast package was updated to lower the overnight POPS for
the smaller island and change the showers to light showers.

Satellite imagery shows widely and randomly dispersed low clouds
mainly across the island's windward waters. They are moving from
east to west with a moderate to strong trade wind flow. Appearance
of these clouds suggests stable stratocumulus with a few
embedded light showers, especially upwind of the Big Island. Both
the GFS and ECMWF, points to some light precip activity over
windward Big Island through the weekend, so kept some scattered
light nighttime showers in the grids for that area. There is a
strong low level inversion at around 5k feet area- wide. This low
level inversion is forecast to hold through Sunday, which means
the continuation of the nice weather like today, and limiting the
light showers. It should be noted, that a patch of radar returns
now exiting Kauai County is false returns and not real

About 280 miles N of Kauai is surface ridge of high pressure.
This ridge extends from a 1022 mb high pressure cell located 670
miles NW of Kauai. The two continues to supply the islands with
sometimes windy trades. The center the high pressure will be
moving east, passing N of the islands on Sunday with central
pressure of 1023 mb. It is'nt a very strong high but will be
strong enough to produce locally strong trades, felt especially in
the more sensitive near-shore waters around Maui County and
waters south of the Big Island. See the marine section below for
more details. In the mean time, the trades will continue to slowly
trend down. It looks like, based on the latest GFS model from this
evening, the trades will lower to below Small Craft Advisory
strength by Tuesday, with our winds becoming light and variable
Monday night. A front is expected to be near the far NW offshore
waters Monday night. Twelve hours later, the front weakens to
where light northerly winds forge ahead of the actual front, which
is just NW of Kauai, to over Kauai and Oahu. The front will be
accompanied by some scattered showers as it work its way down the
island chain. Expect the front to be east of the Big Island by
Wednesday evening. Moderate to strong trades fill in behind the
front along with a cool and dry air mass.


A deep-layer ridge centered N of the islands will change little
through Saturday, with stable trade wind weather prevailing. The
moderate to locally fresh ENE trade winds will deliver stable low
clouds that will favor windward areas, and a subsidence inversion
based near 5500 feet will keep any rainfall very light. While
brief and ISOL MVFR ceilings may occur, VFR conditions will prevail
statewide. AIRMET Tango for low-level turbulence to the S and W
of the mountains will remain posted into at least Saturday.


Trade winds are expected to ease some tonight and Saturday, with
the typically windier areas near Maui and the Big Island continuing
to see Small Craft Advisory (SCA) level winds. The current
northwest swell continues to bring elevated seas, maintaining SCA
level seas for other coastal waters near Molokai through tonight.

With the trade winds easing, expect surf along east facing shores
to lower. The High Surf Advisory (HSA) for east facing shores was
thus cancelled.

A new northwest swell will arrive late in the day Saturday, and
rise rapidly to warning levels. It is expected to peak Sunday,
and then subside some on Monday. An even larger northwest swell
will build late Monday, once again peaking at warning levels.
Exposed coastal waters will once again rise to over 10 feet as
these swells come rolling through.

Expect the current SCA to once again be expanded to most, if not
all, the coastal waters for the first half of the new week as the
new swells arrive.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.


Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 AM HST Saturday for Kaiwi Channel-
Maui County Windward Waters-Maui County Leeward Waters-Big
Island Windward Waters.



H Lau/Birchard/Kinel

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office